In many cases, small auxiliary power unit gas turbines are required to start at very high altitudes, e.g., on the order of forty thousand feet or more. It is known that large main propulsion gas turbines cannot start at such high altitudes and, therefore, this requirement for small auxiliary power unit gas turbines is most demanding, particularly since it's generally accepted that combustion is very difficult at best at such altitudes and most particularly so in small scale applications. Furthermore, small auxiliary power unit gas turbines cannot afford the sophisticated fuel injector and combustor designs customary in large main propulsion gas turbines.
Customarily, combustion experts have believed that chemical kinetics is the dominant parameter of concern at very high altitudes. This ordinarily isn't true which means that incorrect correlations have oftentimes been used to provide improper solutions. Typically, for very high altitudes, the principal problem in terms of combustion or ignition resides in poor fuel atomization.
Generally speaking this is true because the fuel droplet size increases with increases in altitude. The high "g" forces which are experienced, particularly in swirl combustors, cause these large fuel droplets to essentially centrifuge onto the combustor walls thereby adding to the problems associated with the normally poor fuel atomization. As a result, it has been difficult at best to achieve adequate combustion under such circumstances.
In view of the foregoing, it has remained to provide an entirely satisfactory solution to such well-known problems in small turbine engines. It has, thus, been a goal to provide a turbine engine, particularly a small auxiliary power unit, which is capable of very high altitude starts but with a simple, inexpensive design. In response to this situation, the present invention is directed to overcoming such problems by providing a very high altitude turbine combustor.